The present invention is directed to a water-soluble or water-dispersible normally tacky pressure sensitive adhesive.
In papermaking and printing processes the ends of the paper rolls are spliced together to enable the formation of endless paper webs. Splicing of paper within a roll is also undertaken when the paper is cut to remove defective portions. For these purposes, it has been customary to use a repulpable pressure sensitive adhesive-based splicing tape. As various portions of the paper roll or web are cut out and recycled to the paper-making process, it is important for any adhesive which is used in the splice to be readily water-soluble or repulpable (i.e., readily disperses or dissolves in water during a pulping process) so as to not contaminate or discolor the paper which is produced upon being recycled. It is also important for the adhesive splice to be strong enough to resist web failure during the papermaking process or printing process as well as the ability to withstand elevated processing (such as printing) temperatures that may be encountered.
Off-machine paper coating processes also require the use of splicing tapes that exhibit the requisite tack, temperature resistance, and humidity resistance. It would also be desirable for such splicing tapes to exhibit repulpability. However, to date, few commercially acceptable splicing tapes exist that meet the stringent demands of off-machine coaters which subject the tape to excess amounts of water during the coating process as well as elevated temperatures. Also, to date, no commercially-acceptable repulpable splicing tape has been formed that may be satisfactorily employed in off-machine coaters.
Splicing tapes also frequently contribute to a failure in the splice in the paper web as a result of the combination of the speed of the web and the fact that the spliced web is eventually collected in the form of a roll and the splice is caused to conform to the curved surface of the roll. Such prior art splicing tapes employ an adhesive that is sufficiently inelastic such that the splice is incapable of withstanding the rigors encountered during rolling of the web. Such splicing failures contribute to significant inefficiency and expense, as such splice failures must be repaired.
A need also exists for pressure sensitive adhesive labels which can be easily removed from containers or other surfaces to which the adhesive-backed label has been applied. This need exists in particular with respect to containers made from plastic or glass which are intended to be reused. It is accordingly desirable to provide labels which are easily removable by means of water, which labels are most preferably water-soluble or dispersible.
Various attempts have been made to solve such problems as discussed in U.S. Pat. Nos. 3,441,430; 3,556,835; 3,660,147; 4,413,080; 4,413,082; 4,992,501; 5,084,348; 5,094,912; 5,098,962; 5,102,733; 5,125,995; 5,141,810; 5,183,841; and 5,196,504. Such alternative solutions require the use of added tackifying agents or result in adhesives which are soluble in aqueous alkaline solutions. However, the prior art reliance upon water-soluble tackifying agents to impart the required water-solubility for the pressure sensitive adhesive (especially in repulpable adhesives) results in several disadvantages. The prior art relies on the ability of the tackifier to be removed from the adhesive composition in the presence of water (with the intended result that the composition cannot serve as an adhesive due to loss of tack). However, under conditions of high humidity the tackifier may migrate from the adhesive to the adjacent substrate (such as the paper roll). The paper may, as a result, be spotted or discolored by the migrating tackifier. In order to avoid such a consequence, repulpable adhesive tapes are generally sealed in bags to protect against excess humidity. This packaging requirement is costly and burdensome, and as yet unavoidable.
An additional disadvantage that exists with regard to tackified repulpable pressure sensitive adhesives is the fact that the tackified base polymer is itself non-water soluble. As a result the potential for contamination due to the presence of the non-water soluble adhesive exists once the tackifier is solubilized.
Thus, a need exists for a pressure sensitive adhesive which exhibits enhanced water-soluble or water-dispersible characteristics without the need for added tackifying agents and whose water-solubility may be taylored for specific applications. It is also desirable to provide a repulpable water-soluble or water-dispersible pressure sensitive adhesive which is resistant to loss of adhesive properties upon exposure to conditions of high humidity and high temperature, and which does not serve as a source of contamination or discoloration. It is also desirable to provide a high performance pressure sensitive adhesive that is also water-soluble or water-dispersible.